The present invention relates in general to the construction of riflescopes, and in particular to a new and useful riflescope arrangement which incorporates one or more gradient index lenses.
Optical riflescopes are widely used to improve the accuracy of rifles and other small arms. A riflscope, which is a specialized telescope, includes an eye piece section which is closest to the shooter, an objective section which is furthest away from the shooter, and a relay section which is positioned between the objective and eye piece sections. The objective, relay and eye piece sections lie along the optical axis of the riflescope. A tube or housing encloses the three sections and includes mounting arrangements for fixing the riflescope to the firearm. Adjustments are provided to adjust the optical axis to intersect the barrel axis of the fire arm at a selected range. This is known as parallax adjustment. A reticle is also incorporated into the relay section and includes adjustment screws or the like which are accessible on the outer surface of the riflescope tube, for adjusting the aiming point of the rifle.
As with all telescopes, riflescopes must be designed to avoid conventional optical problems such as chromatic aberrations, focusing on distant objects, and other requirements of an optical system.
These problems are generally solved by including additional optical elements or lenses in the system to provide a sufficient number of degrees of freedom to make all adjustments. This technique is not particularly acceptable for riflescopes which must remain small and light weight in design since they must be carried along with the firearm. Additional optical elements also introduce additional problems such as alignment, complexity and increased susceptibility to damage. These are problems which should be avoided in a riflescope which will normally be subjected to greater mechanical and environmental stresses than other telescopes.
Conventional optical systems are generally assembled from a plurality of spherical optical lenses made of glass or other transparent material having a constant index of refraction throughout the material of the element.
Other types of optical elements are known however. Optical elements having aspheric surfaces are known for example which have non-spherical curvatures. This produces a different index of refraction for light as the light passes through the element at different radial positions outwardly from the optical axis of the element.
Gradient index elements are also known where the index of refraction of the material itself changes in a known manner. Radial gradient optical elements are known where the index of refraction changes in the radial direction outwardly from the optical axis of the element. Axial gradient index lenses are also known where the index of refraction changes in the axial direction along the optical axis. Examples of gradient index lenses are known to the skilled artisan in the field. See for example U.S. Pat. Nos. 3,936,149; 4,668,053; 4,674,843; and 5,029,994.